46 SECTIONAL ADDRESSES. 
Resonance and Ionisation Potentials. 
The results of investigations on the absorption spectra of zinc, 
cadmium, and mercury, and on the resonance and ionisation potentials 
of these elements, have shown that for this group of elements the 
ionisation potentials are given ty V = hv/e,where vis the frequency 
denoted by (n, S), namely, that of the last member of the series 
v=(n,8)—(m,P)." It is also known that their resonance potentials are 
given by the same relation with v having the value (,S)—(2,p.) the 
frequency of the first member of a combination series. In the 
case of the alkaline earths similar relations obtain. With the alkali 
elements the frequencies that determine the resonance and ionisation 
potentials are given by v = (n,o)—(n,7)“ andy = (n,6) respectively. 
It is, therefore, clear from the characteristics of the spectral terms 
involved that, while the electron concerned in phenomena associated 
with resonance and ionisation potentials must be the one that is most 
easily displaced in or removed from the atom, this electron must he 
bound in atoms of the elements mentioned—when these are in their 
normal state—in orbits of the 7, type, i.e. in orbits for which the 
subordinate quantum number has the value 1. Now, a reference to 
Table I. will show that this characteristic is exactly the one possessed 
by the electron that is last bound in the atoms of the elements cited. 
It foliows, then, that if we know the type of orbit occupied by the 
last bound electron in the normal atom of any element we can at once 
deduce the type of the series whose first and last members will enable 
us to calculate the resonance and ionisation potentials of the element. 
Moreover, the wave-lengths of such a series will be the ones that will 
be selectively absorbed by the vapour of the element, provided its tem- 
perature is sufficiently low to ensure that the atoms constituting the 
vapour are in their normal state. 
Previous to the publication by Bohr of the scheme in Table I. it 
had been thought that for all elements the resonance and ionisation 
potentials should be obtainable from spectral frequencies of the 
(n, ©) —(m,7) or (n, S)—(m, P) type. Numerous attempts were made 
by investigators of the absorption spectra of such elements as thallium, 
lead, tin, &c., to group the wave-lengths of the radiation absorbed into 
a principal series that would enable one to calculate the critical potentials 
for these elements. These efforts, however, ended in failure, for though 
wave-lengths were found that were selectively absorbed by the vapours 
of the elements referred to, and though it was found possible to fit 
these partially at least into series, it was clear that the series obtained 
did not satisfy the conditions demanded by series of the principal type. 
By the publication of Bohr’s scheme of atomic orbits, however, it 
became evident that since in the case of the aluminium group of 
elements, for example, the electron last acquired in making the atoms 
neutral is bound in an orbit of the n, type, the first member of the 
18 According to Bchr’s scheme n has the value 4 for Zn, 5 for Cd, and 6 
for Hg, while m has the value 2. 
14 Tn this formula n has the value 2 for Li, 3 for Na, 4 for K, 5 for Rb, and 
6 for Cs. 
